I'm looking for (and struggling to find) a comparison for any e-aircraft vs. ground EV modes (car, coach, rail) carrying an equivalent payload, an equivalent distance.

So basically, if you fly from London-Edinburgh in an e-aircraft with 4 passengers... how much electrical energy would that consume vs. driving a car with equivalent passengers and luggage? so a kWh/kg-km comparison.

The plot in this article shows that on an emissions (proportional to fuel burn) basis for combustion engines that you are far better to drive (if multiple passengers), or take the coach or train vs. a domestic flight. https://www.bbc.co.uk/news/science-environment-49349566

I imagine that this relative efficiency comparison only gets worse when you switch from all combustion to all electric, as I understand aircraft are more heavily penalised by battery and electrical system volume/weight than ground transport... and the fact batteries don't burn-off mass as you dissipate energy as per jet fuel. So I'd expect the relative efficiency gap to widen.

Also e-aircraft (whether hydrogen- or battery-electric) are very limited on range/payload so for the same number of passengers it won't be able to fly as far: hence it will spend more journey time in take-off/climb than in cruise and this will make efficiency/kg-km worse again?

Would be very helpful understanding if this logic is flawed - and even more helpful if you have a legitimate quotable reference I can use!

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    $\begingroup$ First, design and field a fleet of all electric passenger aircraft. You can't run a comparison against something that does not yet exist. $\endgroup$
    – WPNSGuy
    Aug 20 at 17:57
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    $\begingroup$ Should the building of runways and roads be factored into the comparison? $\endgroup$ Aug 20 at 23:23
  • $\begingroup$ @WPNSGuy agreed, although we're seeing a lot of CEOs of start-ups winning huge investment for e-aircraft which they claim will "decarbonise flying" (they won't any time soon as they can't compete on energy density vs. jet fuel) and claiming flying short hops in small e-aircraft is a green way to get around vs. the ground. So this latter claim is what I want to think through: short of waiting another decade to finally see the product spec. $\endgroup$ Aug 21 at 10:30
  • $\begingroup$ @Peter Kampf ideally you want all emissiojs factored in but I'd be happy with just the vehicle operational emissions as a starting point. Rail and certainly road networks are already pretty well established across the planet. $\endgroup$ Aug 21 at 10:31
  • $\begingroup$ Another problem with comparing electric aircraft to electric cars is that an aircraft will be using a large fraction of its maximum power at cruise, while a car uses only a small fraction at steady speeds on the level. Almost all of the horsepower is used only for brief acceleration - the oft-cited 0-60 time. $\endgroup$
    – jamesqf
    Aug 21 at 18:12

Train wins, every single time. Zero induced drag.

Thanks to the magic of Timken roller bearings (in truth, plain bearings worked even better when rolling fast enough for the hydrodynamic layer to establish itself: roller bearings are only better at starting), the train's rolling resistance is effectively zero when comparing to any other mode of transportation.

And electrification is easy and 19th century tech.

Because the fixed guideway allows fixed electricity pickup*, which means you don't need to solve the battery problem.

That also means railway stock is dog simple, and does not need elaborate and difficult-to-manufacture battery packs involving exotic metals. Just plain old steel and copper will suffice. That's important because mining and refining exotic metals and making batteries has its own carbon cost. Also, railcars last 50 years - longer than batteries.

In fact there is nothing on an electric train that can burn, except luggage, and furnishings if poorly chosen. That in turn makes extended tunnel construction more feasible.

With rail, rolling resistance is basically nil, which makes the "weight penalty" very low. Also, it does not need to flex (and spall) any rubber tires - another carbon-heavy consumable not needed.

Unfortunately, this efficiency has made rail inefficient. Since there is little weight cost, little effort is made toward making trains light-weight. Indeed, heaviness is desirable for accident survivability. That mass has an accel/decel cost. It is possible for smaller land vehicles to beat trains on kWh/person-km. (But your criteria is kWh/ton-km).

But the heaviness swings both ways: it can effortlessly move immense quantities of cargo.

Full disclosure: There is one more efficient means by kWh-ton-km: cargo ship. But those do not lend themselves to electrification because they venture so far from support. Further, they have serious problems with speed, and require a transload: they cannot bring the cargo directly to the point of use, as trains can.

In a Mad Max no-oil scenario, it is certainly possible to run the ships on massive solar arrays. But that is only going to slow them down much more than they already are. Wind has millennia of prior art, but it doesn't scale well (partly due to square-cube law problems which also affect solar)... so it couldn't add a whole lot for large modern ships like Ever Given.

* Simple matter of suspending a steel catenary wire over the track, which suspends a 4/0 copper contact wire ($20/metre with the Cu shortage) with hangers set so the copper wire is level. This is hung from poles and guyed side arms (simple stuff again). For high speed operation, every km or so, it is tensioned by pulleys and weights. Bonus points if you use 1750 ACSR steel cored aluminum wire as the catenary wire, which then doubles as low resistance feeder.

  • $\begingroup$ Actually, where practical, ocean ship beats train beats truck/lorry beats smaller automobile beats almost all airplanes for energy efficiency measured in payload distance per Joule (or any other measure that compared energy per payload mass and distance). $\endgroup$
    – Zeiss Ikon
    Aug 20 at 18:52
  • $\begingroup$ Conceded, but the human cost starts to pinch. Take the Thomas W. lawson, crew of 18, call it 750 TEU, and best wind speed 16 kts. Say half that in practice. Shenzhen-Long Beach is over a month's sail. $\endgroup$ Aug 20 at 23:27
  • $\begingroup$ Yeah if long-range cargo the best is clearly a ship. Thank you very much for this incredibly detailed overview of the efficiency of trains! Of course, the issue is that many places have limited rail infrastructure - so would be awesome to get a similar detailed description for planes vs. coaches vs. (multiple occupancy) cars. $\endgroup$ Aug 21 at 10:33

For airplanes and cars, one approach might be to break it down to wattage per seat mile for each conveyance. Cars only need about 20-30 HP to cruise on the highway; say 22 kW. An airplane needs about 120 kW (roughly 160 HP) to have decent performance for takeoff and climb with 4 adults and a bit of luggage, and might cruise using between 50 and 75% of that.

Say 50% power, at a high efficiency airspeed (fairly slow), so 60-70 kW for cruising from A to B, about 3 times what the car needs. Maybe going 2 times faster than the car at its most efficient cruising speed, so the tripled power consumption is mitigated by the doubled speed, but the car still wins on a pure efficiency seat mile basis.

Of course this assumes you have a battery that achieves the power density of gasoline, which is some way off. Currently you get about 1/3rd the range of gas, pound for pound, with current electric Li-ion battery power.

  • $\begingroup$ Consider what could be achieved though: an Eta glider weighs 8kN and has a sink rate of 0.4m/s at 100kph so 3200W, which compares favourably with, say, a 100cc motorcycle that achieves the same speed with around double the power. Of course, the glider is converting potential energy in the form of altitude and doesn’t carry an additional energy source or propulsion system, and the infrastructure needed for Etas as personal transport would be nontrivial, but it goes to show that heavier-than air flight isn’t a priori less efficient than other methods. $\endgroup$
    – Frog
    Aug 21 at 21:49

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